The present invention relates to a multi-cylinder internal combustion engine for a motorcar, and particularly to an exhaust manifold thereof having a good performance to keep exhaust in high temperature.
A cast exhaust manifold has thick walls and is heavy, therefore it hinders lightening an internal combustion engine or a vehicle body to be mounted with the engine and has an inconvenience that on starting, exhaust is cooled because of large heat capacity of the manifold to lower an initial performance of an exhaust cleaning catalyst.
In order to maintain the initial performance of the exhaust cleaning catalyst, it is necessary to keep the exhaust at a temperature higher than a predetermined temperature. To cope with this, the exhaust passage may be covered with inner and outer double walls to keep temperature of the exhaust by an insulating air layer between the inner and outer walls.
Japanese Utility Model Publication No. Hei 4-1292 discloses an exhaust manifold based on the above principle.
In this exhaust manifold, upper stream ends of a plurality of cylindrical guide pipes are connected to respective holes of an exhaust port side flange, a manifold case is attached to the exhaust port side flange at an upper stream end and to an exhaust pipe connecting flange at a lower stream end, and the guide pipes are merely arranged in the manifold case with a space between.
The upper stream end of the cylindrical guide pipes is fitted into the hole of the exhaust port side flange and fixed thereto, but the lower stream end of the cylindrical guide pipe is merely arranged freely so as to move also in a direction perpendicular to a longitudinal direction of the guide pipe. As a result, the cylindrical guide pipe vibrates due to vibration of the internal combustion engine or periodic vibration of exhaust pressure in the manifold case, the cylindrical guide pipe comes into contact with the manifold case due to an external force such as gravity or acceleration force, and vibrations and noises occurring at the cylindrical guide pipe are radiated through the manifold case into the atmosphere to reduce vibration-proof and sound-proof characters of the exhaust manifold.
In addition, by contact of the cylindrical guide pipe with the manifold case, heat in the exhaust is radiated from the cylindrical guide pipe through the manifold case into the atmosphere to lower the exhaust temperature.
Since the exhaust port side flange and the exhaust pipe connecting flange are connected by the manifold case only, strength and rigidity of the exhaust manifold main body are low so that the exhaust pipe connecting flange has to be supported by the internal combustion engine or the vehicle body mounted with the engine.